A fibre belt.

And you see here a picture of a tissue section.

So I went ahead, collected wall tissue, prepared it so it was erect, sectioned it, put it on slides and then stuck it under the microscope to have a look, fully expecting to see crossed helices of collagen of some variety.

He's spent over 3,000 hours at that microscope of his, made hundreds of drawings, over 5,000 tissue sections.

The equivalent dose in each tissue or organ is multiplied by a tissue weighting factor, and the sum of these products over the whole body is called the effective dose.